# Understanding the modulation concept

In my research work, I came across the concept in the paper

Y. Hu, P. Wang, Z. Lin, M. Ding and Y. -C. Liang, "Machine Learning Based Signal Detection for Ambient Backscatter Communications," 2019 IEEE International Conference on Communications (ICC), Shanghai, China, 2019, pp. 1-6, doi: 10.1109/ICC.2019.8761796.,

eq. (5) and (6), comparing to reference [2] of that paper, G. Yang, Q. Zhang and Y. Liang, "Cooperative ambient backscatter communications for green internet-of-things", IEEE Internet of Things Journal, vol. 5, no. 2, pp. 1116-1130, April 2018.

"modulation alphabet set of QPSK signal" i.e., $$s(n) \in \mathcal{A}_s$$, where $$n = 1,2,...,256$$, $$s(n)$$ is QPSK signal and $$\mathcal{A}_s$$ is modulation alphabet set.

I am not getting what does this modulation alphabet set mean.

Any help in this regard will be highly appreciated.

• Please properly cite the context of this. I think you're misquoting, because the alphabet of a QPSK doesn't have 256 elements, so we can just speculate how a 256-element alphabet happens. Why speculate? You have the source, so tell us, and we can actually qualifiedly answer. Jul 4 at 12:08
• Voting to close as need of detail, by the way, because you promised to update your previous question with context (so, the very same thing that's missing here, context), and then never did; I can take back that vote as soon as context/citation is given. Jul 4 at 12:09
• Sorry Sir, I didnt give citation as I thought that would increase the work of experts in going to that citation.... But now I am giving it: "Machine Learning Based Signal Detection for Ambient Backscatter Communications" , In this paper pls check equation 5 and 6. And for "modulation alphabet set" we need to refer reference [2] of this paper... Jul 4 at 14:51
• I've added the full reference (what you did really was not a citation…) to the question. Jul 4 at 15:22
• Hm, reading that paper, $\mathcal A_s$ is really just the 4-element modulation alphabet of QPSK. Is that what you're asking about? Are you familiar with QPSK? Jul 4 at 15:33